Hunga Tonga–Hunga Haʻapai

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Hunga Tonga–Hunga Haʻapai () is a submarine volcano in the South Pacific located about 30 km (19 mi) south of the submarine volcano of Fonuafoʻou and 65 km (40 mi) north of Tongatapu, Tonga's main island. It is part of the highly active Kermadec-Tonga subduction zone and its associated volcanic arc, which extends from New Zealand north-northeast to Fiji, and is formed by the subduction of the Pacific Plate under the Indo-Australian Plate. It lies about 100 km (62 mi) above an active seismic zone. The volcano rises around 2,000 m from the seafloor and has a caldera which on the eve of the 2022 eruption was roughly 150 metres (490 ft) below sea level and 4 kilometres (2.5 mi) at its widest extent. The only major above-water part of the volcano are the twin uninhabited islands of Hunga Tonga and Hunga Haʻapai, which are respectively part of the northern and western rim of the caldera. As a result of the volcano's eruptive history, the islands existed as a single landmass from 2015 to 2022: they were merged by a volcanic cone in a volcanic eruption in 2014–2015, and were separated again by a more explosive eruption in 2022, which also reduced the islands in size. The Hunga Tonga–Hunga Haʻapai volcano has seven historical recorded eruptions. The most recent eruption, in January 2022, triggered a tsunami that reached the coasts of Japan and the Americas, along with a volcanic plume that soared 58 km (36 miles) into the mesosphere. It was the largest volcanic eruption since the 1991 eruption of Mount Pinatubo and the biggest explosion recorded in the atmosphere by modern instrumentation, far surpassing any 20th-century volcanic event or nuclear bomb test. NASA determined that the eruption was "hundreds of times more powerful" than the atomic bomb dropped on Hiroshima. It is believed that the 1883 eruption of Krakatoa is the only eruption in recent centuries that rivaled the atmospheric disturbance it produced. The January 2022 eruption is the largest volcanic eruption in the 21st century.

Hunga Tonga–Hunga Haʻapai is a submarine volcano in the Kermadec-Tonga Ridge in South Pacific, a ridge formed by the convergent boundary where the Pacific Plate is subducted by the Indo-Australian Plate, forming a long volcanic and island chain. Hunga Tonga–Hunga Haʻapai volcano lies almost completely underwater, with the exception of two small volcanic islands, Hunga Tonga and Hunga Haʻapai. They are, respectively, the remnants of the northern and western rim of the volcano's caldera. The volcano's base at the seafloor is approximately 20 km in diameter, rising roughly 2,000 m towards the sea surface. Before the 2022 eruption, the volcano's caldera was roughly 150 m below sea-level, and had a size of 4 × 2 km. Its northern and southern portions were filled by volcanic deposits from previous eruptions. The caldera is believed to have been formed by this eruption. Before the 2015 eruption, the two subaerial islands were about 1.6 km (0.99 mi) apart, and were each about 2 km (1.2 mi) long. They are both composed largely of andesite and layered tephra deposits, with steep rocky cliffs. This andesite tends to be of the basaltic type. The 2009 eruption arose from two vents located to the south and northwest of Hunga Haʻapai. The tephra deposited around each vent became connected to the island and nearly tripled its size, but such deposits disappeared with erosion in the following months. Bathymetric data indicate the growth of lava domes along the western caldera rim and mass movements on the northwestern rim between 2008 and 2016. The volcano is also frequently recorded by hydrophones when it generates acoustic noise underwater.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.